Many hormonal steroids undergo an enterohepatic circulation, ultimately followed by fecal or urinary excretion. The structure of the biliary steroids differ markedly from the structures of steroids present in feces and urine. Sporadic reports in the literature suggest that the molecular alteration occurs in the gut and our preliminary experiments indicate that they are mediated by the anaerobic intestinal flora. The proposed research project has two main purposes. First, to isolate and identify the bacterial strains capable of deconjugating and/or altering the hormonal steroids entering the alimentary tract whether through biliary excretion or by oral administration. Experiments will be performed to determine how variations in the intestinal environment may influence the conversion. The second purpose of the project is to identify the steroid metabolites with special emphasis on 21-dehydroxylation. After incubation with bacteria the steroid metabolites will be extracted, purified and identified by TLC, GLC, or IRS. Quantitative and kinetic experiments will be conducted with radioisotopically labelled steroids. The proposed study is a necessary preliminary for further investigations on the biological activities of the bacterially altered hormonal steroids e.g., residual hormonal effect, carcinogenesis, their absorption and their further fate, restoration of the original hormone, progressive metabolism and ultimate excretion.